Every battery has a certain amount of internal resistance. As the battery ages, that resistance value increases a small amount each year. Eventually it gets too high, and the battery has lost too much capacity. For two batteries in parallel, the stronger one will get dragged down by the weaker one, and the overall capacity is no better than the weaker one times 2. If his "old" battery is 10 months old, and has been well cared for, he will likely be just fine doing that, provided his new one is the same make and model. I would add, the "new condition" resistance value is dependant on the physical construction inside the battery, and each manufacturer has a slightly different build structure. That's why it's best to have the pair be from the same manufacturer. For critical applications, some people will go the effort to measure a bunch of new batteries from the same manufacturer for internal resistance, and select a pair which are a close a possible to each other for the internal resistance.